It is known that κ opioid receptor agonists are useful as therapeutic agents for various types of pain. Of them, a κ opioid receptor agonist having high selectivity to a peripheral κ opioid receptor is expected as a medicine producing no side effects on the central nervous system. As such a peripherally-selective κ opioid receptor agonist, synthetic pentapeptides have been reported (Patent Literatures 1 and 2).
Of the synthetic pentapeptides, a compound represented by the following formula (A):
is useful as a therapeutic agent for pain. As a method for producing this compound, Patent Literatures 1 and 2 mentioned above describe a solid-phase peptide synthesis method.